1. Field of the Invention
This invention relates to an electron gun of the type used principally in cathode-ray tubes.
2. Description of the Prior Art
A conventional prior art triode gun found in cathode-ray tubes consists of a cathode 2, a grid electrode 4 and an anode or accelerating electrode 6, as shown in FIG. 1. For discussion, the cathode is taken as a reference and is assumed to be at zero potential. The anode is at a positive potential and the grid is at a negative potential. This structure forms an electron beam 8 whose current can be controlled by varying the potential of the grid electrode, usually extending over a range from zero voltage for maximum beam current to a certain negative cut-off voltage for no beam current. A modification of this conventional gun is a tetrode gun, in which the anode is followed by another positive electrode, as in U.S. Pat. No. 2,276,320 to Linder and U.S. Pat. No. 2,939,703 to Niklas. This modification is not relevant to this discussion of prior art.
An important characteristic of an electron gun, especially of those used in television tubes, is the variation of the electron beam current I.sub.b with grid voltage V.sub.g, or with "drive voltage", defined as V.sub.d = V.sub.g - V.sub.gc where V.sub.gc is the cut-off voltage.
A mathematically rigorous analysis for I.sub.b = f(V.sub.d), taking into account the gun geometry, space-charge effects and aberrations, is only possible by means of computers. Several approximate methods have been used to derive relations of the type EQU I.sub.b = K V.sub.d .sup..alpha.V.sub.gc .sup..beta.
With exponents varying between 3/2 and 7/2. Measurements on experimental triodes by various investigators appear to confirm the theoretical predictions. The exponent .alpha. is usually referred to as the "gamma" of the characteristic. The value of gamma is fixed in conventional electron guns. By "fixed", it is not meant that it is absolutely constant, because the gamma is probably a slowly varying function of V.sub.d and is only approximately or substantially constant. However, the value of gamma is fixed in the sense of being predetermined in a conventional tube by the nature of the tube itself.